The present invention relates to a process for producing a incompatible polymer-containing polyester film. More particularly, it relates to a process for producing a incompatible polymer-containing polyester film, in which a substantially undried polyester is melt extruded with a incompatible polymer from a vented twin-screw extruder under the specific conditions, so that the incompatible polymer can be uniformly dispersed in the polyester while sufficiently maintaining inherent qualities of the polyester and the incompatible polymer. The process of the present invention is also energy-saving and capable of reducing the production cost.
Since polyester films have excellent properties, those have been widely used in recent years for electronic white boards or magnetic cards such as credit cards, prepaid cards, for example, telephone cards, JR (Japan Railway) cards, pachinko (pinball) cards, etc., in many fields of commercial activities such as finance, communication, amusement, transportation, etc., and their range of utilization is expected to expand in the future.
Recently, the aromatic polyester-based white films containing a white pigment such as titanium oxide or barium sulfate, or those having fine closed cells provided for reducing weight and bettering cushioning characteristics have come to be used for various types of printing material and image receiving sheet.
Various methods have been proposed for forming the fine closed cells. For example, there are mentioned methods in which a gas or a gasifiable material is incorporated in the film and let it expand as described in Japanese Patent Application Laid-Open (KOKAI) No. 50-38765, Japanese Patent Publication (KOKOKU) No. 57-46456 and Japanese Patent Application Laid-Open (KOKAI) No. 57-34931; a material which is chemically decomposed to generate a gas is added and the film is expanded as described in Japanese Patent Application Laid-Open (KOKAI) No. 52-43871 and Japanese Patent Publication (KOKOKU) No. 58-50625; a liquid is impregnated in the film after molding and a material soluble in the solvent is extracted as described in Japanese Patent Publication (KOKOKU) No. 52-27666.
These methods, however, have problems in film productivity and thickness uniformity, so that as means for forming the cells in the film during or after the stretching step, usually a method comprising blending a thermoplastic resin incompatible with polyesters, such as proposed in Japanese Patent Application Laid-Open (KOKAI) No. 63-168441, is used.
The polymers incompatible with polyesters, include polyolefin polymers such as polyethylene and polypropylene, and polystyrene polymers which include polystyrene homopolymers and random copolymers composed of repeating units of mainly styrene and other monomers.
Polyacrylate-based, polyacryl-based and silicone-based polymers are also usable as incompatible polymer.
Various problems are generated in the extrusion method in which a polyester and a incompatible polymer are plasticized and melt extruded into a film by using a same extruder.
Since the thermoplastic resins incompatible with polyesters are different in melting point from the polyesters, plasticization time also differs from polyesters to the thermoplastic resins incompatible with polyesters (for instance, the melting point of polypropylene is about 160.degree. C., so that it is plasticized earlier than the polyester in the extruder), which causes such problems as improper charge of material and variation of pressure at the delivery end of the extruder.
Particularly when such improper extrusion takes place in a single-screw extruder, the plasticized resin adheres to the surface of the extruder screw, thereby blocking transfer of the material and its plasticization. Once such problems arise, it is necessary to dismount the extruder screw by stopping the production line for removing the resin adhering to the screw, so that the productivity is badly deteriorated.
In the case of a twin-screw extruder, the problem of imperfect plasticization also occurs because the sites of plasticization of the two polymers differ due to the difference in melting point. This may cause agglomeration of the unmelted polyester particles in the molten polymer extruded into a film. In a twin-screw extruder having vacuum vents, removal of moisture may become insufficient because of improper plasticization of the polyester in vacuo, resulting in an excess decrease of degree of polymerization of the polyester.
It is known that when a polyester is melt-extruded in the presence of moisture, the degree of polymerization lowers because of hydrolysis of the polyester. In order to prevent the decrease of the degree of polymerization thereof, methods such as a method of guiding the polyester to the extruder after reducing its water content by drying, after the crystallization treatment are taken, but the problem still exists that the polymerization degree may lower due to the action of moisture present in the incompatible polymer blended in the polyester in the extruder, or the amount of the incompatible polymer to be blended can not be increased because of the presence of moisture mentioned above.
The said method of guiding the polyester to the extruder after reducing water content of the polymer incompatible with the polyester necessitates equipment for drying the incompatible polymer, which requires extra equipment investment. Also, when the polymer is subjected to the drying treatment by heating, there may take place coloration due to heat deterioration of the said polymer or a decrease of polymerization degree, and a reduction of drying temperature is required for avoiding such a problem. Low melting point is also detrimental to efficient drying of the polymer.
As a result of the present inventor's earnest studies to solve the above problems, it has been found that these problems can be overcome by specifying the incompatible polymer content, total output per unit time, extruder screw speed and cylinder bore to satisfy the specific conditions by using a specific extruder. The present invention has been attained on the basis of the above finding.